The technologies employed in manufacturing pipes is as diverse as the types of liquids carried therethrough. Some pipes require a very high tensile strength to withstand the internal forces exerted thereon. Other pipes are required to be fabricated with special materials to resist corrosion due to the corrosive liquids and gases carried therein. Yet other pipes require optimum manufacturing efficiency to maintain the product marketable in a highly cost competitive marketplace. As can be seen, there are many constraints placed on pipe makers to compete in the diverse market place.
The manufacture of clay pipes was initiated more than a thousand years ago, and continues today. The fabrication of pipes from clay remains an on-going technology, as clay pipes are cost effective, they have a very long lifetime, exceeding a hundred years, and the base material remains plentiful. Vitrified clay pipes are a type of pipe that is formed from clay, or a mixture of clay and shale soil, and then fired at a temperature of about 2,000 degrees F. At this high temperature the clay mineral particles fuse into an inert and stable material that is durable and resists corrosion from many liquids and gasses. Some ancient cities in the Mediterranean sea area still use vitrified clay sewer pipes that were fabricated and installed over two thousand years ago.
Unlike some types of pipe material, such as ductile steel or plastic, vitrified clay pipes are more brittle and cannot be handled in a manner to expose the pipes to large impact forces, otherwise the vitrified clay material can crack or break. Insofar as many pipe systems, especially sewer systems, require a gas and liquid tight system, a crack or break in the pipe system can result in either an expensive repair or replacement of the broken pipe.
Clay pipes, like many other types of pipes adapted for use in sewer systems, include a male/female connection to mate the pipes together and form a gas and liquid seal therebetween. The male end of the pipe is often referred to as the spigot end, and the female end of the pipe is referred to as a bell. The spigot end of the pipe is inserted into the bell end to form a joint that maintains alignment of the pipes when subjected to external loads, such as settling earth or loads placed thereon due to dirt, heavy equipment, structures, roads, etc. An elastomeric seal is often placed between the spigot and bell to seal the two pipes together and prevent the escape of liquids or gasses therefrom even when pressurized, and under extreme changes in ground temperature conditions.
Clay pipes are individually handed during the many stages of manufacturing, transporting and installation. Each pipe must be moved, lifted and lowered into position a number of times between manufacture and installation. Since the bell end of the clay pipe is larger in diameter than the remainder of the pipe, it is the structure that is more subject to breakage. When a clay pipe is laid on a flat surface, such as a manufacturing plant floor, a truck bed or a open cut trench at the installation site, it is the bell end of the pipe, and the spigot end that contacts with the surface. The middle tubular part of the pipe is above the surface of the ground. The spigot end of the pipe is generally wrapped with a polyester resin or polyurethane molded casting to achieve an accurate diameter. Thus, the spigot end of the clay pipe is not as susceptible to damage from breakage. Point loading forces on the bottom of the bell can cause damage, especially if the pipe is subject to an impact, such as a short drop. In this instance, the compressive force, especially on the bell, may exceed the compressive strength of the clay material and break it. It is understood that the round shape of a clay pipe contributes, in part, to the strength of the pipe. The compressive strength of vitrified clay is relatively high, as compared to the tensile strength.
When two clay pipes are mated together, with the spigot end of one pipe inserted into the bell end of an adjacent pipe, excessive differential external loads on either pipe can cause the bell to crack or break from the resulting force exerted on the bell. If an excessive downward force is applied to the spigot end of the mated pipe, then the bottom portion of the bell of the other pipe will experience a force that may overcome the tensile strength of the bell. The bell can thus crack or break. Alternatively, if an excessive downward force is applied to the bell end of the mated pipe, then the top of the bell may experience a force that is greater than the tensile strength of the bell, and thus crack or break it. Either of these situations can occur if too much force, or an uneven compaction force, is applied to one or the other of the mated pipes during installation, backfilling, during subsequent settlement, and from live loads. The foregoing is a result of the characteristic of the relative low tensile strength of clay pipes.
Another situation is frequently encountered in which the tension force on the bell end of a clay pipe is exceeded. When the spigot end of a clay pipe, with an elastomeric seal therearound, is forced into the bell end of an adjacent pipe, the bell experiences a radial outwardly directed force. The tensile strength of the bell can be exceeded if the pipes are not accurately aligned when forced together. This situation is also exacerbated in cold weather when the elastomeric seal is less pliable and the force to mate the pipes together must be increased.
Clay pipes are manufactured by an extruding process in which wet clay is pushed into a mold having a cavity the shape of the pipe to be formed. The bell part of the mold is at a remote end of the mold, and thus it is difficult to push the wet clay back into the complex-shaped bell portion of the mold. The extruded clay is then placed in a dryer to reduce the moisture content of the molded clay prior to placing it into a kiln where it is heated to a vitrification temperature, resulting in a hardened clay pipe. Because of the inherent extruding process, it is difficult to fabricate a clay pipe with an extended or elongated bell. An axially longer bell is highly desirable, as there is more surface area for sealing with the spigot end of another pipe. With a longer bell, the spigot can slide out of the bell a certain extent and still form a seal with the inner surface of the bell. An extruded bell becomes relative less strong as it is lengthened axially.
From the foregoing, it can be seen a need exists for a pipe that has a bell with increased tensile strength. A need also exists for a pipe that has an extended bell, thus optimizing the sealing constraints with a spigot end of another pipe. Another need exists for a pipe having a bell end that is less brittle than the clay material itself, thus enabling the pipe to withstand higher impact forces.